Refinery process furnaces are widely used to heat hydrocarbons in a variety of services, for example, crude oil feed to an atmospheric tower, crude residuum from the atmospheric tower for feed to a vacuum tower, and the like. Perhaps the most severe service is the heating of feedstock to a delayed coker. While coke deposition can be a problem in any refinery process furnace, because of the high temperatures employed and the residual nature of the coker feedstock, there is a pronounced tendency for the formation of coke deposits on the inside wall of the radiant tubing through the coker preheat furnace.
Regardless of service, the formation of coke deposits is not desirable. Coke deposits can lead to increased pressure in the tubes due to the restriction of flow, and to higher tube wall temperatures due to the insulative effects of the coke deposits. Both higher pressure and higher temperature lead to premature failure of the tubes. Furthermore, it is often necessary to periodically remove the tube from service and remove the coke deposits by burning off the deposited coke by oxidation with air or another oxidant that is passed through the tube at a high temperature. This periodic burn-off can result in severe thermal cycling which also reduces the life of the tube.
One factor that has been identified as contributing to high coke formation rates and high tube metal temperatures is the presence of heat flux imbalances. See Martin, G. R., "Heat-Flux Imbalances in Fired Heaters Cause Operating Problems," Hydrocarbon Processing, pp. 103-109 (May 1998). Heat-flux imbalances can be caused by many factors, such as, for example, furnace design and furnace operating conditions, including such things as the ratio of radiant section height to width, burner-to-tube distances, number and type of burners, flame shape, air preheat and air temperature, radiant section tube layout, one or more burners out of service, insufficient air to burners, fuel gas composition, burner fuel pressure, higher-than-normal firing rates, fouled burner tips, eroded burner tip orifices, insufficient draft, and the like.
In multi-pass heating arrangements, the tube layouts in existing heaters are generally different for each pass, i.e. each pass is positioned in a different place in the furnace. A pass located at the bottom of the furnace will see a flame temperature of 3000-3500.degree. F. (1650-1930.degree. C.) near its outlet, but a pass located near the arch at the top of the radiant section will see much lower temperatures. One way to compensate for heat-flux imbalances is to control the relative rates of feedstock supplied to each pass so that the outlet temperatures are about the same. However, this still does not avoid the existence of hot spots in individual tube passes that can lead to localized coke deposition.
An improvement over wall-mounted tube runs in the radiant section is the double-fired heater in U.S. Pat. No. 5,078,857 to Melton. This uses a bank of tubes running centrally through the firebox with a row of burners on either side thereof. The tubes pass through slots in the end wall of the radiant section, and an insulated header cover encloses the conventional returns. Unfortunately, this tube design still does not allow the placement of multiple passes within a single firebox that have the same heating pattern and fluxes in each pass.
It would be desirable to have available a multi-pass furnace design which allows the various passes to have about the same heating pattern and heat fluxes. It would also be desirable to increase the effective tube-heating surface of each pass, while at the same time reducing the pressure drop through the tubing. Further, it would also be desirable to eliminate the need for insulated end-tube header boxes and provide an improved manner of supporting the horizontal tubes. The present invention addresses these and other needs in the refinery process furnace art.